Process for producing Bi- and Pb-containing oxide superconducting wiring films

ABSTRACT

In an oxide superconducting film wiring, when the line width is reduced, the evaporation of a component during firing becomes so vigorous that it becomes impossible to form a desired single crystal phase, which causes a significant lowering in the properties of the oxide superconducting wiring. This problem can be solved by preventing the evaporation of the evaporable component during the firing. Examples of this include a process wherein plate is placed above the superconductor forming material film wiring pattern on the substrate so as to face each other, the plate comprising a material having no chemical influence on the superconducting wiring, and a pattern of a material containing an evaporable component is arbitrarily formed, a process wherein a pattern having a smaller line width is sandwiched between patterns having a larger line width, and a process wherein the firing atmosphere or the concentration of the evaporable component in the pattern is varied depending upon the line width.

BACKGROUND OF THE INVENTION

The present invention relates to a superconducting film, and moreparticularly, to a process for producing a film of a bismuth basedperovskite superconducting material containing lead.

A superconducting film wiring currently practicable in the art is aBi-base superconducting film wiring.

Examples of the Bi-base superconducting film wiring realizable in theart include that comprising a 10 K phase having a superconductingtransition temperature, i.e., a critical temperature, Tc, of 10 K atwhich a normal conducting phase is transferred to a superconductingphase, that comprising a 80 K phase having a critical temperature, Tc,of 80 K, and that comprising a 110 K phase having a criticaltemperature, Tc, of 110 K, but it is known in the art that it is verydifficult to prepare a Bi-based superconducting film wiring of a 110 Kphase.

In the superconductor field, however, since a high superconductingtransition temperature greatly benefits the cooling, such as a reductionof size of the whole equipment through a simplification of a coolingdevice, a Bi-based superconducting wiring of a 110 K phase is required.

DESCRIPTION OF THE RELATED ART

In general, a Bi-based superconducting film has been produced bydepositing a material film on a substrate by sputtering, vacuumevaporation or the like and subjecting the material film to a post-heattreatment to synthesize a superconducting film. In this case, thepatterning is usually conducted either by using a mask in the depositionof the superconducting material film or by conducting etching after thedeposition.

A very precise control of the temperature is necessary for producing aBi-based superconducting film comprising a 110 K phase, and theformation of a single phase of a 110 K phase without the addition of Pbto the composition has been regarded as very difficult. As the presentinventors have already reported (see "Appl. Phys. Lett., 54", pp.1362-1364 (1989)), however, since Pb vigorously evaporates duringsintering, the amount of Pb becomes insufficient even in the case of afilm having wide pattern width, and thus it is not easy to form aBi-based superconducting film comprising a 110 K phased.

The present inventors have succeeded in forming a Bi-basedsuperconducting film having a single phase of a 110 K phase by adding Pbin a considerably higher concentration than that in the case of a bulkor the like (see "Appl. Phys. Lett., 55", pp. 1252-1254 (1989)).

In general, in a Bi-based superconducting film, there exists asuperconducting phase wherein the critical temperature, Tc, variesdepending upon the difference in the number of Cu-O planes contained inthe unit cell. At the present time, in the Bi-based superconducting filmrepresented by the formula Bi₂ Sr₂ Ca_(n-1) Cu_(n) O_(x), threesuperconducting compounds are known, i.e., a phase having a Tc of 10 Kwherein n is 1, a phase having a Tc of 80 K wherein n is 2, and a phasehaving a Tc of 110 K wherein n is 3.

The synthesis of a superconducting film comprising a 110 K phase havingthe highest critical temperature, Tc, is expected from the practicalviewpoint, but as described above, even in the case of a film having awidth pattern width, the Pb vigorously evaporates during sintering andreached a state such that the amount of Pb becomes insufficient. Forthis reason, a technique wherein a large amount of Pb is added has beendeveloped. In this case, however, it has been found that the evaporationof Pb becomes more vigorous in the case of a pattern having a small linewidth, so that the proportion of the formation of the 110 K phase isdecreased. When a larger amount of Pb is added to a pattern having asmall line width, the superconducting material film partially meltsduring sintering to form a 110 K phase. The partial melting temperatureis closely related to the proportion of Pb. Specifically, when theproportion of Pb is high, the superconducting material film unfavorablymelts at a low temperature of about 840° C. and the melting becomesvigorous. This is liable to cause variations in the composition fromplace to place, and consequently, a Tc phase or a crystal having a smallgrain diameter is often formed, and thus the texture becomesheterogeneous. This causes the critical current density, Jc, and thecritical temperature, Tc, to be lowered, and thus favorable results cannot be obtained.

Accordingly, an object of the present invention is to form asubstantially single phase of a 110 K phase through the prevention of anevaporation of a large amount of an evaporable component such as Pb inan early stage of the firing and formation of a heterogeneous texture bythe use of a very simple technique in the firing of a film having asmaller width pattern of a Bi-based superconducting material containingPb.

SUMMARY OF THE INVENTION

To attain the above-described object, the present invention provides aprocess for producing a superconducting film, comprising the steps of:forming on a substrate a film wiring pattern of a material capable ofproducing a superconducting material upon being fired; and firing thewiring pattern of the superconductor forming material film whilepreventing or compensating for the evaporation of an evaporablecomponent (hereinafter referred to as "easily evaporable component").

The term "superconductor forming material" is intended to mean amaterial capable of becoming a superconducting material upon being firedand is an aspect including a superconducting material per se.

The specific means for preventing or compensating for the evaporation ofthe easily evaporable component of the superconducting material may be,for example, a first, to arrange a plate above a superconductor formingmaterial film wiring pattern on a substrate so as to face each other,the plate comprising a material having no chemical influence on thesuperconducting wiring;

second, to form a film of a material containing an easily evaporablecomponent (hereinafter referred to as "material containing an easilyevaporable component") on the surface of the plate into a pattern,preferably a pattern corresponding to a superconductor forming materialfilm wiring pattern;

third, to form a film pattern of a material containing an easilyevaporable component in a larger width than that of the superconductorforming material film wiring pattern along and on both sides of thesuperconductor forming material film wiring pattern on the substrate;

fourth, to place a material containing an easily evaporable componentwithin a firing oven for firing a superconductor forming material filmwiring pattern;

fifth, to feed a vapor of an easily evaporable component within thefiring oven for firing the superconductor forming material film wiringpattern;

sixth, to separate a wiring pattern of a superconductor forming materialfilm into a wiring portion having a larger pattern width and a wiringportion having a smaller pattern width, and placing the wiring portionhaving a larger pattern width and the wiring portion having a smallerpattern width in a different firing atmosphere; or

seventh, to differentiate the concentration of the easily evaporablecomponent in the wiring portion having a larger pattern width from thatof the wiring portion having a smaller pattern width in the wiringpattern of the superconductor forming material film, so that theconcentration of the easily evaporable component in the wiring portionhaving a smaller pattern width is higher than that in the wiring portionhaving a larger pattern width.

In a preferred embodiment, the superconducting material is aBi-Pb-Sr-Ca-Cu-O-base perovskite superconducting material, and theeasily evaporable component is Pb. In this case, the superconductorforming material is a Bi-Pb-Sr-Ca-Cu-O-base material comprising Bi, Pb,Sr, Ca and Cu in a Bi:Pb:Sr:Ca:Cu molar ratio of preferably (1.9 to2.1):(1.2 to 2.2):2:(1.9 to 2.2):(3 to 3.5), more preferably (1.9 to2.1):(1.5 to 1.8):2:(1.9 to 2.2):(3 to 3.5). The general composition ofthe Bi-Pb-Sr-Ca-Cu-O-base perovskite superconducting material thusprepared is represented by the formula (Bi_(1-x) Pb_(x))₂ (Sr_(1-y)Ca_(y))₄ Cu₃ O_(z) wherein 0<x<1, 0<y<1 and 0<z.

In the present invention, a particularly large effect can be attainedwhen the wiring width is 1 mm or less, more preferably 0.5 mm, mostpreferably 0.3 mm. Although the present invention can be applied to botha thin film and a thick film, a larger effect can be attainedparticularly when the film is a thin film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a firing temperature profile of a Bi-Pb-Sr-Ca-Cu-O-baseperovskite superconducting material;

FIGS. 2A and 2B are X-ray diffraction patterns of samples firedaccording to the temperature profile OabB shown in FIG. 1;

FIG. 3 is a diagram showing a change in composition in the step offiring a material (a ground plane layer and a signal layer) firedaccording to a temperature profile shown in FIG. 1;

FIG. 4 is a quasi-binary equilibrium phase diagram of PbO-CaO;

FIGS. 5A, 5B and 5C are X-ray diffraction patterns of samples of thepresent invention and comparative samples prepared in Example 1;

FIG. 6 is a diagram showing a change of the electrical resistance withthe temperature for a sample prepared in Example 1;

FIG. 7 is a cross-sectional view of a firing oven used in Example 2;

FIG. 8 is a diagram showing a change of the electrical resistivity withthe temperature for a sample prepared in Example 2;

FIG. 9 is a wiring pattern of a sample prepared in Example 4;

FIGS. 10A and 10B are X-ray diffraction patterns of samples prepared inExample 4;

FIG. 11 is a diagram showing a change of the electrical resistivity withthe temperature for a sample prepared in Example 4;

FIG. 12 is a cross-sectional view of a firing oven used in Example 5;

FIG. 13 is an X-ray diffraction pattern of a sample prepared in Example5;

FIGS. 14A and 14B are respectively a cross sectional view of a firingoven used in Example 6 and an enlarged view of a substrate holdingportion of the firing oven shown in FIG. 14A;

FIGS. 15A and 15B are X-ray diffraction patterns of samples prepared inExample 6;

FIG. 16 is a diagram showing a change of the electrical resistivity withthe temperature for a sample prepared in Example 6;

FIGS. 17A and 17B are X-ray diffraction patterns of samples prepared inExample 7;

FIG. 18 is a diagram showing a change of the electrical resistivity withthe temperature for a sample prepared in Example 7; and

FIG. 19 is an X-ray diffraction pattern of a comparative sample preparedin Example 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An oxide superconducting film having a critical temperature of 100 K orabove can be formed by any of the thick film method and the thin filmmethod, but when the line width is about 1.0 mm or less, it becomesdifficult to form the 110 K phase as a single phase and the 80 K phaseas well is present, so that the critical temperature is rapidly lowered.In this case, the material often exhibits no superconducting state evenat a liquid nitrogen temperature (77 K) The present inventors considerthat the reason why it becomes difficult to form the 110 K phase whenthe line width is reduced resides in the evaporation of a componenthaving a particularly high vapor pressure, among the componentsconstituting the superconductor.

Specifically, with respect to a Bi-Pb-Sr-Ca-Cu-O-based material whichthe present inventors have studied for practical use, a component havinga particularly high vapor pressure, and having a great influence on theproperties is PbO, which exhibits the relationship between thetemperature and the vapor pressure as shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Temperature (°C.)                                                                      Vapor pressure (Torr)                                         ______________________________________                                        about 680       1 × 10.sup.-2                                           about 750       1 × 10.sup.-1                                           about 930       .sup. 1 × 10.sup.0                                       about 1100     .sup. 1 × 10.sup.1                                      1470 (b.p.)     760 (1 atm)                                                   ______________________________________                                    

Since the temperature at which firing is conducted for crystallizationto form a superconducting phase is about 850° C., the vapor pressure ofPbO is so high that an evaporation easily occurs at this stage.

In particular, the reason why it becomes difficult to form the 110 Kphase as a single phase in a small pattern width in question is believedto be because, since the partial pressure of PbO attributable to theevaporation from the periphery including the PbO itself decreases with areduction in the line width, the evaporation of the component is notinhibited and the exposed surface area per unit volume becomes large,which causes the amount of evaporation of PbO to become large.

Accordingly, in the present invention, a 110 K phase is formed as asubstantially single phase even in a small line width, through theprevention or compensation for the evaporation of particularly PbOduring firing, to thereby produce an oxide superconducting film having ahigh critical temperature.

The first means for preventing the evaporation of an easily evaporablecomponent is to spatially limit the firing atmosphere around the wiringportion. Specifically, a plate (a substrate) of a material having nochemical influence on a superconducting film, for example, MgO, Al₂ O₃,LaAlO₃, sapphire, SrTiO₃, ZrO₂ (including a stabilized or partiallystabilized element), LaGaO₃, MgAl₂ O₄, Y₂ O₃, SiO₂, 2MgO.SiO₂, Si,MgO.SiO₂ or a quartz glass is disposed so as to face a substrate havinga wiring pattern of a superconductor forming material formed thereon.The gap between the wiring pattern on the wiring substrate and thecounter plate (counter substrate) is preferably narrow, and is generally1 mm or less, preferably about 0.5 mm.

The second means is to form a pattern of a material containing an easilyevaporable component on the counter substrate. This enables the vaporpressure of the easily evaporable component from the superconductingwiring pattern limited by the counter substrate to be maintained at ahigh level such that the evaporation of the easily evaporable componentfrom the wiring pattern of the superconductor forming material film isfurther prevented. Further, when the counter substrate having thispattern formed thereon is heated to a temperature equal to or above thetemperature of the substrate on which a superconducting wiring patternis to be formed, the evaporation of the easily evaporable component fromthe counter substrate is accelerated. In this case as well, the gapbetween the wiring substrate and the pattern of the counter plate ispreferably as narrow as possible, i.e., preferably 1 mm or less, morepreferably about 0.5 mm.

The pattern of the material containing an easily evaporable componentmay be formed on the whole surface of the counter substrate. In apreferred embodiment, however, preferably the configuration of thepattern corresponds to the superconducting wiring pattern fired. This isbecause, in the formation of a multilayer structure etc., preferablythat no excess evaporated component is deposited on a portion other thanthe superconducting wiring pattern of the substrate. Further, theevaporated component might accumulate within the oven, to thus make itimpossible to control the atmosphere in the oven. In this sense, saidfirst means is preferred because no deposit occurs on the countersubstrate, and there is no pattern of a material containing an easilyevaporable component.

The material containing an easily evaporable component for forming apattern on the counter substrate may be a material comprising the samecomponent or composition as that of the superconducting wiring material.For example, in the case of a Bi-Pb-Sr-Ca-Cu-O-based superconductor, useis made of a Bi-Pb-Sr-Ca-Cu-O-based material. In theBi-Pb-Sr-Ca-Cu-O-based superconductor, the easily evaporable componentsare Bi, Pb, Cu, etc. In this case, Pb is particularly evaporable, and atthe same time, plays an important role in the formation of a 110 Kphase.

In the case of a thick film having a thickness of 20 μm or more, when aprinted wiring pattern of a superconductor forming film is fired, if thematerial is maintained for a short time at a temperature above apredetermined firing temperature to an extent such that the evaporationof PbO does not vigorously occur and the temperature is returned to thepredetermined firing temperature to conduct firing, the growth of acrystal grain is accelerated, and a dense film having no gap in thegrain boundary can be obtained.

The third means is that when a wiring pattern of a superconductorforming material film is formed on a substrate, a pattern of a materialcontaining an easily evaporable component, the line width of the wiringpattern being larger than that of the wiring pattern of thesuperconductor forming material film, is formed along and both sides ofthe wiring pattern of the superconductor forming material film.Specifically, when the line width of a superconductor wiring pattern is1 mm or less, particularly 0.5 mm or less, it becomes difficult to forma 110 K phase. On both sides of such a narrow wiring pattern of asuperconductor forming film is provided a pattern of a materialcontaining an easily evaporable component in a larger line width thanthat of the wiring pattern of the superconductor forming material film,generally 1 mm or more, for example, 2 mm or more, further 3 mm or more,while leaving a gap of preferably 1 mm or less, more preferably 0.5 mmor less between the narrow wiring pattern of a superconductor formingfilm and the pattern of the material containing an easily evaporablecomponent.

Since the pattern of a material containing an easily evaporablecomponent is formed on the same substrate as that of the wiring patternof the superconductor forming material film, it is preferred that anidentical composition be used for the material containing an easilyevaporable component and the superconductor forming material, and thefilm formation and the patterning for the material containing an easilyevaporable component be conducted simultaneously with the film formationand the patterning for the superconductor forming material. In thiscase, since both the patterns formed by the firing comprise asuperconducting film, a device may be made on the superconducting wiringpattern so that a pattern having a larger line width (for example, aground line) is disposed on both sides of a pattern having a smallerline width (for example, a signal wiring), thus causing the largerwiring pattering provided on both sides of the smaller wiring pattern tobe left in the final product. Since, however, the superconducting wiringis used in applications such as a high electron mobility transistor(HEMT) and a circuit wherein use is made of a Josephson element, when anexcess pattern exists in close vicinity of the line at the time oftransmission of a radio frequency signal, the excess pattern exhibitsthe same function as that of the earth and lowers the quality of asignal. Therefore, such an excess pattern should be removed. The excesspattern can be removed by wet etching (with an aqueous solution ofhydrochloric acid, phosphoric acid or the like) or dry etching (areactive ion etching or the like).

The fourth means is to feed a vapor of an easily evaporable componentinto a firing oven or to place a material containing an easilyevaporable component (in the form of a pellet or the like) within afiring oven to feed a vapor of the material containing an easilyevaporable component into the firing atmosphere.

The fifth means comprises dividing the superconducting wiring patterninto a wiring portion having a larger line width and a wiring portionhaving a smaller line width and firing the wiring portion having alarger line width in an atmosphere different from that used in thefiring of the wiring portion having a smaller line width. Since thewiring layer having a larger line width and the wiring layer having asmaller line width are fired in respective separate atmospheres, it ispossible to select respective firing atmospheres so that the amounts ofevaporation of high vapor pressure components in respective wiring layercompositions become substantially equal to each other. This enables thecompositions of the respective wiring layers after the firing to becoincided with each other, and predetermined superconducting propertiesto be simultaneously obtained in all wiring layers.

For example, the wiring portion having a larger line width and thewiring portion having a small line width may be separated from eachother in an identical firing chamber by means of a partitioning memberto make the atmospheres of the two wiring portions different from eachother. The following method is particularly convenient. A wiring portionhaving a larger line width is formed on one principal surface of asubstrate, and a wiring portion having a smaller line width is formed onthe other principal surface of the substrate. The substrate is held on athrough hole provided on a member for partitioning the firing chamberinto two regions. Both sides of the substrate are respectively exposedto the two regions on both sides of the partition, and the through holeis hermetically sealed by the substrate, thus enabling the atmospheresof the two regions to be made different from each other.

The sixth means is to differentiate the concentration of the easilyevaporable component in the wiring portion having a larger line widthfrom that of the easily evaporable component in the wiring portionhaving a smaller line width, so that the concentration of the easilyevaporable component of the wiring portion having a smaller line widthbecomes higher. That is, the concentration of the evaporable componentis previously regulated depending upon the line width or theevaporability. In this case, when the superconductor is aBi-Pb-Sr-Ca-Cu-O-based superconductor, the Pb/Bi molar ratio ispreferably from 0.6 to 1.0 in any of the wiring patterns to be fired.

An experiment was conducted on how the Pb concentration of aBi-Pb-Sr-Ca-Cu-O-based superconductor wiring pattern changes with thewiring width during the firing and the confirmation of the formed phaseat that time.

A Bi-Pb-Sr-Ca-Cu-O layer was deposited on a MgO single crystal substrateby RF magnetron sputtering and fired to form a Bi₂ Sr₂ Ca₂ Cu₃ O_(x)oxide superconductor wiring. In a deposit composition comprising Bi, Pb,Sr, Ca and Cu in a molar ratio of 1.0:0.9:1.0:1.0:1.7, a whole groundplane layer having a size of 10 mm×10 mm and a signal layer having aline width of 1 mm were deposited, and firing was conducted according toa temperature profile shown in FIG. 1. In FIG. 1, the OdD is atemperature profile used in the actual firing. In this case, thetemperature is raised from room temperature to 805° C., maintained atthat temperature for 20 min, further raised to 855° C., maintained atthat temperature for one hour and lowered to room temperature. The OaA,ObB and OcC are each a temperature profile wherein the firing isdiscontinued without continuation to the final stage. The firing wasconducted in the air. After the firing, the film was subjected tomeasurement of an X-ray diffraction and analysis of the composition.

The X-ray diffraction patterns of samples fired according to the profileOabB are shown in FIGS. 2A and 2B. In the drawings, L, LL and Prespectively represent peaks attributable to a 80 K phase, a 10 K phaseand Ca₂ PbO_(x). As shown in FIG. 2A, in a ground plane layer samplehaving a size of 10 mm×10 mm, a strong peak attributable to Ca₂ PbO_(x)is observed, whereas as shown in FIG. 2B, in a signal layer samplehaving a line width of 1 mm, a peak attributable to Ca₂ PbO_(x) is veryweak.

The results of the analysis of the composition conducted in each stageof the firing with respect to a sample of a ground plane layer and asample of a signal layer are shown in FIG. 3. In both cases, therelative concentration of each component is plotted by taking theconcentration of Sr concentration as 1. The Bi, Cu and Ca concentrationsare substantially constant from the deposited state through each stageof the firing. On the other hand, it is apparent that the Pbconcentration decreases with the advance of the firing. In particular,in samples fired according to the temperature profile OabB correspondingto FIGS. 2A and 2B, the signal layer having a smaller width (line width:1 mm) exhibits a more rapid reduction in the Pb concentration than theground plane layer (10 mm×10 mm) having a larger width signal layer,□◯Δ: ground plane layer).

FIG. 4 is a quasi binary equilibrium phase diagram of PbO-CaO, and it isapparent that when the concentration ratio of PbO to CaO is larger than1:2, partial melting occurs at 815° C., and accordingly, it is expectedthat, in more a complicated Bi-Pb-Sr-Ca-Cu-O-based superconductor also,the melting occurs at least partially at a temperature equivalent to orbelow that temperature. Therefore, it is substantially certain thatmelting occurs when a large amount of PbO stays within the film in thecourse of the temperature rise from 805° C. to 855° C. according to theactual firing temperature profile OabcdD shown in FIG. 1 and the holdingof the temperature at 855° C.

EXAMPLE 1

A superconductor forming material was deposited by radio-frequencymagnetron sputtering on a MgO single crystal substrate covered with ametal mask having a linear pattern, to form a superconductor formingmaterial film pattern having a line width of 0.5 mm (500 μm) and athickness of 0.8 μm and comprising Bi, Pb, Sr, Ca and Cu in a ratio of1.0:0.8:1.0:1.0:1.6.

Then, the substrate having a superconductor forming material film formedthereon was fired in the air at 850° C. for one hour to form a Bi-basedperovskite superconducting film.

In the firing, as Example 1, (a) a MgO substrate was covered with a FGA(fine grained alumina) plate and the distance between the MgO plate andthe FGA plate was set to about 1 mm, and as comparative examples, (b)firing was conducted in a state such that the distance between the FGAplate and the MgO substrate was set to about 5 mm, and (c) no FGA platewas used.

The X-ray diffraction patterns determined on samples (a), (b) and (c)thus prepared are respectively shown in FIGS. 5A, 5B and 5C. In thesediagrams, H and L represent a 110 K phase and an 80 K phase,respectively.

From FIG. 5A, it is apparent that main peaks in the X-ray diffractionpattern of the sample (a) are H, i.e., attributable to the 110 K phase.On the other hand, from FIGS. 5B and 5C, it is apparent that main peaksin the X-ray diffraction patterns of the samples (b) and (c) are L,i.e., attributable to the 80 K phase.

FIG. 6 is a graph showing a change of electrical resistivity withtemperature in a superconducting film wiring having a line width of 0.5mm prepared by firing under the same condition as that used in thepreparation of the sample (a). In FIG. 6, the abscissa represents thetemperature and the ordinate represents the electrical resistivityvalue.

From FIG. 6, it is apparent that the critical temperature is 97 K, i.e.,substantially the same as that in the case of a superconducting filmwiring provided with a pattern having a large line width of 1 mm ormore.

The critical current density at the liquid nitrogen temperature was2×10³ A/cm², i.e., sufficiently large.

EXAMPLE 1 (EXAMPLE OF THICK FILM)

Raw material powders of Bi₂ O₃, PbO, SrCO₃, CaCO₃ and CuO were preparedand mixed with each other so that the molar ratio of Bi:Pb:Sr:Ca:Cu was0.7:0.3:1:1:1.8. The mixed powder was fired at 845° C. for 150 hr toprepare a Bi-Pb-Sr-Ca-Cu-O-based oxide superconductor.

The oxide superconductor was coarsely ground in a mortar and thensubjected to regulation of the grain size in a ball mill. Terpineol wasadded as a viscosity modifier to the powder, and the mixture was kneadedwith acetone as a solvent. The kneaded product was dried to removeacetone, and benzene was mixed with the dried product. The mixture wasdried to regulate the viscosity to prepare a superconducting paste.

A plurality of sheets of a magnesia single crystal substrate having asize of 15 mm square and a thickness of 0.5 mm were provided, and a linepattern having a line width of 0.5 mm and a length of 10 mm was printedby screen printing through the use of this paste.

Then, a firing oven shown in FIG. 7 was provided. Specifically, aluminaplates 8,9 respectively provided with heaters 5,6 at the back thereofwere mounted on a frame 7 comprising a quartz glass, and substrates 2,4were placed opposite each other on the ceramic plates 8,9 though the useof a magnesia spacer 10. The substrate 2 is a substrate for forming asuperconducting wiring, and the substrate 4 is a substrate having apattern for preventing the evaporation of PbO from the superconductingwiring. Thus, the gap between the wiring pattern 1 and the counterpattern 3 was kept at 0.5 mm.

Two sheets of a magnesia substrate having, printed thereon, a conductorline having a line width of 0.5 mm, a thickness of 30 μm and a length of10 mm were mounted in a firing oven shown in FIG. 7 and placed oppositeto each other at a space of 0.5 mm therebetween, and the upper substratewas used as a wiring pattern.

In the air, heaters 5,6 were energized, the two substrates were heatedat 860° C. for 10 min, and the temperature of the upper substrate 2 waslowered to 840° C. with the temperature of the lower substrate 4 beingkept at 850° C. for 6 hr to conduct firing.

The resultant conductor line (superconducting phase) of the uppersubstrate 2 was subjected to a measurement of the temperature dependenceof the electrical resistivity thereof. The results are shown in FIG. 8.The critical temperature was about 98 K.

For comparison, the above-described procedure was repeated.Specifically, raw material powders of Bi₂ O₃, PbO, SrCO₃, CaCO₃ and CuOwere prepared and mixed with each other so that the molar ratio ofBi:Pb:Sr:Ca:Cu was 0.7:0.3:1:1:1.8. The mixed powder was fired at 845°C. for 150 hr to prepare a Bi-Pb-Sr-Ca-Cu-O-based oxide superconductor.

The oxide superconductor was coarsely ground in a mortar and thensubjected to a regulation of the grain size in a ball mill. Terpineolwas added as a viscosity modifier to the powder, and the mixture waskneaded with acetone as a solvent. The kneaded product was dried toremove acetone, and benzene was mixed with the dried product. Themixture was dried to regulate the viscosity to prepare a superconductingpaste.

A line pattern having a line width of 1 mm was printed by screenprinting through the use of this paste, and dried. In the air, theprinted and dried substrate was heated to 860° C. for 10 min, lowered to245° C. and fired at that temperature for 6 hr. The temperaturedependence of the resistivity measured on the line pattern of thissample is also given in FIG. 8, which clearly demonstrates the effect ofthe present invention.

EXAMPLE 3 (EXAMPLE OF THICK FILM)

The procedure of Example 2 was repeated, except that a line patternhaving a line width of 0.5 mm and a length of 10 mm was formed on onemagnesia single crystal substrate and a pattern having a size of 10 mmsquare was formed on another magnesia single crystal substrate by screenprinting. The magnesia substrate having printed thereon a line patternhaving a line width of 0.5 mm, a thickness of 30 μm and a length of 10mm was mounted on the upper part of the firing oven, while the magnesiasubstrate having, printed thereon, a solid pattern having a size of 10mm square and a thickness of 30 μm was mounted on the lower part of thefiring oven.

The firing was conducted in the same manner as that of Example 2. Theresultant conductor line pattern (superconducting phase) was subjectedto a measurement of the temperature dependence of the resistivity. Theresults are shown in FIG. 8. The critical temperature was about 100 K.

EXAMPLE 4

As shown in FIG. 9, a 1 μm-thick thin film of an oxide having acomposition ratio of Bi:Pb:Sr:Ca:Cu of 1.0:0.8:1.0:1.0:1.6 was formed ona magnesia single crystal substrate 13 by radio-frequency magnetronsputtering through the use of a metal mask. On both sides of a line 14having line widths of 0.5 mm (500 μm) and 1 mm were provided patterns15,16 each having a line width of 3 mm while leaving a gap of 0.5 mm(500 μm) between the line 14 and each pattern.

This substrate was fired in the air at 855° C. for one hour to form asuperconducting phase.

X-ray diffraction patterns measured on the resultant lines respectivelyhaving line widths of 0.5 mm and 1 mm are shown in FIGS. 10A and 10B. Inthese drawings also, H and L represent a 110 K phase and an 80 K phase,respectively.

As can be seen from these drawings, main peaks in the X-diffractionpatterns are attributable to the 110 K phase.

The results of measurement of the temperature dependence of theresistivity on a line having a line width of 1 mm are shown in FIG. 11.The critical temperature was 107 K, which was exactly the same as thatin the case of a line having a large line width.

The critical current density at liquid nitrogen atmosphere wassufficiently large and 4×10³ A/cm².

The results where the line width was 0.5 mm and the patterns 15,16 werenot formed on both sides of the line are as shown as Comparative Examplein Example 1. Specifically, as shown in FIG. 5C, main peaks wereattributable to an 80 K phase. The measurement of the temperaturedependence of the resistivity has revealed that, although theresistivity rapidly decreased around 110 K, the temperature at which theresistance became zero (0) was as low as 78 K. Further, at liquidnitrogen temperature, the superconducting state was broken by a smallamount of current.

EXAMPLE 5

In this example, firing was conducted through the use of a tubular ovenas shown in FIG. 12. In the drawing, numeral 21 designates a tubularfiring chamber, numeral 22 a gas feeding pipe, numeral 23 an evacuationpipe, numeral 24 a first heater, numeral 25 a second heater, numeral 26a third heater, numeral 27 a Bi-Pb-Sr-Ca-Cu-O pellet, and numeral 28 asubstrate having, formed thereon, a wiring pattern 29 of asuperconductor forming material. This tubular oven is usually called a"three zone tubular oven" and has three zones which can be independentlysubjected to regulation of the temperature respectively by three pairsof heaters (the first heater 24, the second heater 25 and the thirdheater 26).

In this firing oven, a magnesia single crystal substrate 28 having thesame superconductor forming material film pattern 29 [line width: 0.5 mm(500 μm)] as that prepared in Example 1 was mounted.

A mixed gas (O₂ +N₂) was fed at a flow rate of 5 liters/min into thetubular firing chamber 21 through the gas feed pipe 22, and the settingwas conducted so that the temperature of the substrate 28 became 850° C.during firing. An oxide pellet 27 having a diameter of 30 mm and aheight of 2 mm and containing Bi, Pb, Sr, Ca and Cu in a molar ratio of1:1:1:1:1.5 was placed in the first zone corresponding to the firstheater 24, and the temperature of the first zone was set to 800° C.

Under the above setting conditions, the vapor pressure of PbO around thesubstrate 28 was enhanced to about 10⁻⁵ Torr.

As can be seen from FIG. 13 showing an X-ray diffraction pattern of theresultant sample, a superconducting wiring composed mainly of a 110 Kphase was formed even in a small line width by firing under the aboveconditions.

EXAMPLE 6

Bi-Pb-Sr-Ca-Cu-O was subjected to RF magnetron sputtering to deposit a 1μm-thick ground plane layer having a size of 10 mm×10 mm on oneprincipal surface of a MgO substrate. A metal mask was applied to theother principal surface of the MgO substrate, and deposition wasconducted in the same manner as that described above to deposit a 1μm-thick signal layer having a width of 0.5 mm (500 μm).

Both the above depositions were conducted at a substrate temperature of350° C. so as to form a deposit having such a composition that theBi:Pb:Sr:Ca:Cu molar ratio was 1.0:0.6:1.0:1.0:1.6.

The deposit layers were then fired. The tubular oven used in the firingand a substrate holding portion of the oven are respectively shown inFIG. 14A and 14B.

The tubular oven 31 comprises a quartz glass. The inside of the firingchamber is partitioned into two regions, that is, upper and lowerregions (M, N), by means of a partitioning plate 32 comprising a quartzglass, and a substrate 33 is held in substantially a central portion ofthe partitioning plate 32. The substrate holding portion has an openingpassing through the thickness, and the edge of the opening was preparedin the form of a ring alumina substrate receiver 37 provided with aflange. The outer edge of the substrate 33 was put and held on theflange of the substrate receiver 37, and the outer edge of the substrateand the flange were brought into close contact with each other to blockthe opening to separate the two regions, i.e., upper and lower regions(M, N), from each other. A gas for a firing atmosphere was introducedthrough a gas flow inlet 34 provided at the left end of the tubular oven31, and evacuation was conducted through a gas flow outlet 35 providedat the right end of the tubular oven. A Bi-Pb-Sr-Ca-Cu-O pellet 36having a Bi:Pb:Sr:Ca:Cu ratio of 1.0:1.0:1.0:1.0:1.5 was placed upstreamof the position holding the substrate 33 in the upper region M, and asuitable amount of PbO was fed into an atmosphere gas flow from the flowinlet 34.

The MgO substrate 33 was held in a substrate holding portion of thepartitioning plate 32 within the tubular oven 31 in such a manner thatthe surface having a signal layer deposited thereon faced upward and thesurface having a ground plane layer deposited thereon faced downward. Amixed gas (N₂ +O₂) (flow ratio N₂ :O₂ =4:1, total flow rate=5liters/min) was fed through the gas flow inlet 34. Firing was conductedaccording to a firing temperature profile OabcdD (805° C.×20 min 805°C.×1 hr) shown in FIG. 1.

After firing, the substrate 33 was taken out of the tubular oven 31 andsubjected to X ray diffraction. The results are shown in FIGS. 15A and15B. In the drawings, H and L represents a 110 K phase and an 80 Kphase, respectively. From the results, it is apparent that the groundplane layer (FIG. 15A) and the signal layer (FIG. 15B) in the film afterthe firing each consist essentially of a single phase of the 110 Kphase.

The resultant individual wiring layers were subjected to measurement ofa change of the electrical resistance with the temperature, and theresults are shown in FIG. 16. The critical temperature, Tc, of theground plane layer (a) was exactly same as that of the signal layer (b)and 100 K. In both wiring layers, the critical current density at liquidnitrogen temperature was 4×10³ A/cm², i.e., satisfactory from theviewpoint of practical use.

EXAMPLE 7

A ground plane layer having a size of 10 mm×10 mm was deposited on a MgOsubstrate by RF magnetron sputtering. A metal mask was applied to theopposite surface of the MgO substrate, and deposition was conducted inthe same manner as that described above to deposit a signal layer havinga width of 0.5 mm (500 μm) was deposited.

Both of the above depositions were conducted at a substrate temperatureof 350° C. so as to form a deposit having such a composition that theBi:Pb:Sr:Ca:Cu ratio was 1.0:0.6:1.0:1.0:1.6 in the case of the groundplane layer and 1.0:0.8:1.0:1.0:1.6 in the case of the signal layer.That is, the Pb concentration of the signal layer was higher than thatof the ground plane layer.

The deposit layers were then fired. The firing was conducted within aconventional quartz tubular oven through the use of a mixed gas (N₂ +O₂)(flow ratio N₂ :O₂ =4:1, total flow rate=5 liters/min) as a firingatmosphere according to a firing temperature profile OabcdD (805° C.×20min+855° C.×1 hr) shown in FIG. 1.

After the firing, the substrate was taken out of the tubular oven andsubjected to X ray diffraction measurement. The results are shown inFIGS. 17A and 17B. In the drawings, H represents a diffraction peakattributable to a 110 K phase. From the results, it is apparent that theground plane layer (FIG. 17A) and the signal layer (FIG. 17B) in thefilm after the firing each consist essentially of a single phase of the110 K phase.

The resultant individual wiring layers were subjected to measurement ofa change of the electrical resistivity with the temperature, and theresults are shown in FIG. 18. The critical temperature, Tc, of theground plane layer (a) was exactly the same as that of the signal layer(b) and 100 K. In both wiring layers, the critical current density atliquid nitrogen temperature was 1×10⁴ A/cm², i.e., satisfactory from theviewpoint of practical use.

For comparison, wiring layers was formed in the same manner as that ofthe above Example, except that both the ground plane layer and thesignal layer had a deposit composition having a Bi:Pb:Sr:Ca:Cu ratio of1.0:0.8:1.0:1.0:1.6. The resultant wiring layers were subjected to X-raydiffraction. The results of the X-ray diffraction on the ground planelayer are shown in FIG. 19. In the drawing, H, L and LL representdiffraction peaks attributable to a 110 K phase, an 80 K phase and a 10K phase. It is apparent that the Pb concentration during the depositionof the ground plane layer was so high that the 80 K phase and the 10 Kphase were present as well as the 110 K phase.

According to the present invention, it is possible to form an oxidesuperconducting wiring having a high critical temperature, particularlya Bi-Pb-Sr-Ca-Cu-O based oxide superconducting wiring rich in a 110 Kphase even when the line width is small, which renders the oxidesuperconducting wiring formed by the present invention useful as ahigh-temperature superconducting wiring used at liquid nitrogentemperature in high electron mobility transistors, Josephson elements,etc.

We claim:
 1. A process for producing a superconducting wiring film,comprising the steps of:forming on a substrate a film wiring pattern ofa material capable of producing a Pb- and Bi-containing oxidesuperconducting material upon being fired; and firing the superconductorforming material film wiring pattern while preventing or compensatingfor the evaporation of at least one of Pb and Bi components of thesuperconducting material, wherein the means for preventing orcompensating for the evaporation of the at least one of Pb and Bicomponents of the superconducting material is to place above and closeto the superconductor forming material film wiring pattern on thesubstrate so that the plate and the superconductor forming material filmwiring pattern face each other, the plate comprising a material havingno chemical influence on the superconductor forming material film wiringpattern.
 2. A process according to claim 1, wherein the material of theplate is selected from a group consisting of Al₂ O₃, LaAlO₃, MgO,sapphire, SrTiO₃, ZrO₂, LaGaO₃, MgAl₂ O₄, Y₂ O₃, SiO₂, 2MgO.SiO₂, Si,MgO.SiO₂ and a quartz glass.
 3. A process according to claim 1, whereina film of a material containing the at least one of Pb and Bi componentsof the superconducting material is formed on the surface facing thesuperconductor forming material film wiring pattern of said plate.
 4. Aprocess according to claim 3, wherein the film of a material containingthe at least one of Pb and Bi components of the superconducting materialis formed in a pattern corresponding to the superconductor formingmaterial film wiring pattern.
 5. A process according to claim 3, whereinfiring is conducted while maintaining the film of a material containingthe at least one of Pb and Bi components of the superconducting materialat a temperature equal to ore above the temperature of thesuperconductor forming material film wiring pattern.
 6. A processaccording to claim 3, wherein the superconducting material is aBi-Pb-Sr-Ca-Cu-O-based perovskite superconducting material and the atleast one of Pb and Bi components of the superconducting material is Pb.7. A process according to claim 6, wherein the material containing theat least one of Pb and Bi components of the superconducting material isa Bi-Pb-Sr-Ca-Cu-O-based material.
 8. A process according to claim 1,wherein a gap between the superconductor forming material film wiringpattern and the plate is 1 mm or less.
 9. A process according to claim8, wherein the gap between the superconductor forming material filmwiring pattern and the plate is 0.5 mm or less.
 10. A process accordingto claim 3, wherein a gap between the superconductor forming materialfilm wiring pattern and the film of a material containing the at leastone of Pb and Bi components of the superconducting material is 1 mm orless.
 11. A process according to claim 10, wherein the gap between thesuperconductor forming material film wiring pattern and the film of amaterial containing the at least one of Pb and Bi components of thesuperconducting material is 0.5 mm of less.
 12. A process according toclaim 1, wherein a line width of the superconductor forming materialfilm wiring pattern is 1 mm or less.
 13. A process according to claim12, wherein the line width of the superconductor forming material filmwiring pattern is 0.3 mm or less.
 14. A process according to claim 1wherein the superconducting material is a Bi-Pb-Sr-Ca-Cu-O-basedperovskite superconducting material and the at least one of Pb and Bicomponents of the superconducting material is Pb.
 15. A processaccording to claim 14, wherein the superconducting material is aBi-Pb-Sr-Ca-Cu-O-based material containing Bi, Pb, Sr, Ca and Cu in aBi:Pb:Sr:Ca:Cu molar ratio of (1.9 to 2.1):(1.2 to 2.2):2:(1.9 to2.2):(3to 3.5).
 16. A process according to claim 15, wherein thesuperconducting material is a Bi-Pb-Sr-Ca-Cu-O-based material containingBi, Pb, Sr, Ca and Cu in a Bi:Pb:Sr:Ca:Cu molar ratio of (1.9 to2.1):(1.5 to 1.8):2:(1.9 to 2.2):(3 to 3.5).
 17. A process for producinga superconducting wiring film, comprising the steps of:forming asubstrate a film wiring pattern of a material capable of producing a Pb-and Bi-containing oxide superconducting material upon being fired; andfiring the superconductor forming material film wiring pattern whilepreventing or compensating for the evaporation of at least one of Pb andBi components of the superconducting material, wherein the means forpreventing or compensating for the evaporation of the at least one of Pband Bi components of the superconducting material is to form a filmpattern of a material containing the at least one of Pb and Bicomponents of the superconducting material with a width larger than thatof the superconductor forming material film wiring pattern along and onboth sides of the superconductor forming material film wiring pattern onthe substrate.
 18. A process according to claim 17, wherein thesuperconducting material is a Bi-Pb-Sr-Ca-Cu-O-based perovskitesuperconducting material and the at least one of the Pb and Bicomponents of the superconducting material is Pb.
 19. A processaccording to claim 18, wherein the film pattern of a material containingthe at least one of Pb and Bi components of the superconducting materialis a Bi-Pb-Sr-Ca-Cu-O-based perovskite superconducting material.
 20. Aprocess according to claim 17, wherein the superconducting formingmaterial film wiring pattern is a signal wiring layer, and the filmpattern of a material containing the at least one of Pb and Bicomponents of the superconducting material is a ground plane layer. 21.A process according to claim 15, wherein a gap between the film patternof a material containing the at least one of Pb and Bi components of thesuperconducting material is removed after the firing.
 22. A processaccording to claim 17, wherein a gap between the superconductor formingmaterial film wiring pattern and the film pattern of a materialcontaining the at least one of Pb and Bi components of thesuperconducting material is 1 mm or less.
 23. A process according toclaim 21, wherein the gap between the superconductor forming materialfilm wiring pattern and the film pattern of a material containing the atleast one of Pb and Bi components of the superconducting material is 0.5mm or less.
 24. A process according to claim 17, wherein a line width ofthe superconductor forming material film wiring pattern is 1 mm or less.25. A process according to claim 17, wherein the line width of thesuperconductor forming material film wiring pattern is 0.3 mm or less.26. A process according to claim 18, wherein the superconductingmaterial is a Bi-Pb-Sr-Ca-Cu-O-based material containing Bi, Pb, Sr, Caand Cu in a Bi:Pb:Sr:Ca:Cu molar ratio of (1.9 to 2.1):(1.2 to2.2):2:(1.9 to 2.2):(3 to 3.5).
 27. A process according to claim 26,wherein the superconducting material is a Bi-Pb-Sr-Ca-Cu-O-basedmaterial containing Bi, Pb, Sr, Ca and Cu in a Bi:Pb:Sr:Ca:Cu molarratio of (1.9 to 2.1):(1.5 to 1.8):2:(1.9 to 2.2):(3 to 3.5).
 28. Aprocess for producing a superconducting wiring film, comprising thesteps of:forming on a substrate a film wiring pattern of a materialcapable of producing a Pb- and Bi-containing oxide superconductingmaterial upon being fired; and firing the superconductor formingmaterial film wiring pattern while preventing or compensating for theevaporation of at least one of Pb and Bi components of thesuperconducting material, wherein the means for preventing orcompensating for the evaporation of the at least one of Pb and Bicomponents of the superconducting material is to divide thesuperconductor forming material film wiring pattern into first andsecond wiring portions, the first wiring portion having a line widthlarger than that of the second wiring portion and to fire the firstwiring portion in an atmosphere different from that used in the firingof the second wiring portion.
 29. A process according to claim 28,wherein the firing atmosphere in the second wiring portion has a vaporpressure higher than that of the firing atmosphere in the first wiringportion.
 30. A process according to claim 28, wherein thesuperconducting material is a Bi-Pb-Sr-Ca-Cu-O-based perovskitesuperconducting material and the at least one of Pb and Bi components ofthe superconducting material is Pb.
 31. A process according to claim 28,wherein the width of superconductor forming material film wiring patternis 1 mm or less.
 32. A process according to claim 30, wherein thesuperconducting material is a Bi-Pb-Sr-Ca-Cu-O-based material containingBi, Pb, Sr, Ca and Cu in a Bi:Pb:Sr:Ca:Cu molar ratio of (1.9 to2.1):(1.2 to 2.2):2:(1.9 to 2.2):(3 to 3.5).
 33. A process according toclaim 32, wherein the superconducting material is aBi-Pb-Sr-Ca-Cu-O-based material containing Bi, Pb, Sr, Ca and Cu in aBi:Pb:Sr:Ca:Cu molar ratio of (1.9 to 2.1):(1.5 to 1.8):2:(1.9 to2.2):(3 to 3.5).
 34. A process for producing a superconducting wiringfilm, comprising the steps of:forming on a substrate a film wiringpattern of a material capable of producing a Pb- and Bi-containing oxidesuperconducting material upon being fired; and firing the superconductorforming material film wiring pattern while preventing or compensatingfor the evaporation of at least one of Pb and Bi components of thesuperconducting material. wherein the means for preventing orcompensating for the evaporation of the at least one of Pb and Bicomponents of the superconducting material is to divide thesuperconductor forming material film wiring pattern into first andsecond wiring portions, the first wiring portion having a line widthlarger than that of the second wiring portion and to differentiate theconcentration of the at least one of Pb and Bi components of thesuperconducting material in the first wiring portion from that of the atleast one of Pb and Bi components of the superconducting material in thesecond wiring portion so that the concentration of the at least one ofPb and Bi components of the superconducting material of the secondwiring portion becomes higher than that of the at least one of Pb and Bicomponents of the superconducting material of the first wiring portion.35. A process according to claim 34, wherein the superconductingmaterial is a Bi-Pb-Sr-Ca-Cu-O-based perovskite superconducting materialand the at least one of Pb and Bi components of the superconductingmaterial is Pb.
 36. A process according to claim 35, wherein the molarratio of the Pb concentration to the Bi concentration of theBi-Pb-Sr-Ca-Cu-O-based superconductor forming material film wiringpattern is in the range of from 0.6 to 1.1.
 37. A process according toclaim 34, wherein the width of superconductor forming material filmwiring pattern is 1 mm or less.
 38. A process according to claim 35,wherein the superconducting material is a Bi-Pb-Sr-Ca-Cu-O-basedmaterial containing Bi, Pb, Sr, Ca and Cu in a Bi:Pb:Sr:Ca:Cu molarratio of (1.9 to 2.1):(1.2 to 2.2):2:(1.9 to 2.2):(3 to 3.5).
 39. Aprocess according to claim 38, wherein the superconducting material is aBi-Pb-Sr-Ca-Cu-O-based material containing Bi, Pb, Sr, Ca and Cu in aBi:Pb:Sr:Ca:Cu molar ratio of (1.9 to 2.1):(1.5 to 1.8):2:(1.9 to2.2):(3 to 3.5).